Photolithographic processing techniques are used to create patterned layers for microelectronic structures. Each of the patterned layers can be used as an element of a microelectronic device being fabricated, as a mask for a dopant being provided to the substrate below or for other purposes such as an etch mask.
Typically, a photoresist layer is formed on a layer to be patterned. This photoresist layer is selectively exposed to radiation such as light and then developed to form a patterned photoresist layer. This patterned photoresist layer serves as a mask when etching the layer beneath. By etching the portions of a layer exposed by the patterned photoresist layer, the pattern of the photoresist layer can be transferred to the layer below.
As the line widths of microelectronic devices have been reduced to 0.35 .mu.m and smaller, the limits of resolution and the depth of focus (DOF) available through conventional i-line illumination systems may be exceeded. Accordingly, conventional illumination systems may be inadequate for the production of microelectronic devices requiring relatively narrow line widths. In particular, the line widths required for a 256 M dynamic random access memory have prompted the development of improved illumination systems. For example, illumination systems using KrF excimer lasers have been developed.
A conventional projection exposure system for photolithographic processing is illustrated in FIG. 1. The components of this projection exposure system include a light source 1, a first reflector 3, a shutter 5, filters 7, a second reflector 9, a fly's eye lens 11, an aperture 13, a photomask 15, a third reflector 17, a condenser lens 19, a fourth reflector 21, and an object lens 23. This system is used to project an image of the mask 15 onto the wafer 25. The light generated by the light source 1 is transmitted to the wafer 25 along the light path as illustrated.
A component of the light generated by the light source is transmitted to the wafer using the aperture 13. The aperture can be a quadruple aperture or an annular aperture. Both an annular aperture and a quadruple aperture may have the effects of phase-shift and semi-transmission. The modified illumination transmitted through the aperture 13 includes a component with oblique incidence. This modified illumination may also have a component with vertical illumination. Accordingly, resolution and depth of field may be increased when a pattern is formed. Various apertures for conventional projection exposure systems are illustrated in FIGS. 2-5. The quadruple aperture of FIG. 2 has four symmetrically arranged portions 29 which transmit light. Furthermore, a central portion 27 of the aperture blocks light so that only off-axis illumination is transmitted by the aperture.
An annular aperture is illustrated in FIG. 3. In the annular aperture of FIG. 3, a circular light blocking portion 31 is located in the center of the aperture. A light transmitting portion 33 forms an annulet surrounding the light blocking portion 31. The annular aperture may be generally more effective in the generation of illumination with an oblique incidence than a quadruple aperture.
In addition, a central region 35 of a quadruple aperture can provide a phase-shift of 180.degree. and allow the semi-transmission of light, as shown in FIG. 4. Similarly, a central region 36 of an annular aperture can provide a phase-shift of 180.degree. and allow the semi-transmission of light, as shown in FIG. 5.
A conventional reflector 37 for a projection exposure system is illustrated in FIG. 6. The reflector 37 has a surface 39 for reflecting nearly 100% of the light incident thereon.
According to the conventional projection exposure system, the light transmitted through the aperture 13 to the mask 15 has an oblique incident component. Even though the light intensity may be uniformly controlled by the fly's eye lens 11, diffraction generated by the aperture 13 may result in nonuniform light intensity of the wafer. Accordingly, it may be difficult to generate uniform line widths on the wafer 25.